Ink composition

ABSTRACT

The present invention relates to a radiation curable ink composition comprising an ester compound, that is suitable for printing high gloss images. The invention further relates to a method for making such ink composition and a printing method using such ink composition.

The present invention relates to an ink composition. The presentinvention further relates to a method for preparing an ink composition.In addition, the present invention relates to a method for applying animage onto a recording medium. The present invention also relates to useof an ester compound in an ink composition for obtaining high gloss.

BACKGROUND OF THE INVENTION

Radiation-curable inkjet ink compositions are known in the art. Theseink compositions comprise one or more radiation curable components. Aspecial class of radiation curable inkjet ink compositions are phasechange radiation curable inkjet ink compositions. These inks are fluidat elevated temperature and become solid—even if not yet cured—at lowertemperatures. These inks are typically jetted at elevated temperatures.Phase change inks may become solid or semi-solid upon cooling down on arecording medium, e.g. a sheet of paper. As a result, spread of adroplet of ink on the recording medium may be decreased and colorbleeding may be prevented.

An example of a phase change radiation curable inkjet ink is a gellingradiation curable inkjet ink. Gelling radiation curable inkjet ink maybe jetted at elevated temperature and may undergo a rapid increase inviscosity when being jetted onto a recording medium. Because of theincrease in viscosity, the droplets of ink jetted onto the recordingmedium may not spread much and hence, color bleeding may be preventedeven if the ink composition is not immediately cured after being appliedonto the recording medium. The gelling behavior may be provided byadding a suitable gellant to the radiation curable ink composition.Gelling radiation curable inkjet ink compositions typically comprise agellant. Gellants are also known in the art as gelling agents orthickeners. Examples of gellants used in gelling radiation curableinkjet ink compositions are waxes, such as natural waxes and long chaincarboxylic acids, and ketones. A disadvantage of these gellants is thatimages printed using an ink composition comprising such gellantgenerally show low or medium gloss level, while high gloss is desiredfor images printed using a radiation curable ink. There is a need forgelling radiation curable ink compositions that provide high glossimages.

It is therefore an object of the present invention to provide a gellingradiation curable ink composition that provides high gloss images.

SUMMARY OF THE INVENTION

The object of the invention is achieved in a radiation-curable inkjetink composition comprising a radiation curable component, theradiation-curable inkjet ink composition further comprising an estercompound, the ester compound consisting essentially of a condensationproduct of a first reactant and a second reactant, wherein the firstreactant is a compound A comprising at least 3 first functional groups,and wherein the second reactant comprises at least one compound B,wherein the at least one compound B comprises a second functional group,wherein the first functional group is a first group selected from ahydroxyl functional group and a carboxylic acid functional group and thesecond functional group is a second group selected from a hydroxylfunctional group and a carboxylic acid functional group, wherein thefirst functional group is different from the second functional group andwherein the ester compound is present in an amount of 0.3 wt %-3.0 wt %based on the total weight of the radiation-curable inkjet inkcomposition.

Radiation Curable Medium

The radiation curable inkjet ink composition may comprise a radiationcurable medium. The radiation curable medium may comprise at least oneradiation curable component. A radiation curable component is acomponent that may react (e.g. polymerize) under influence of suitableradiation, such as electromagnetic radiation, e.g. ultraviolet (UV)radiation. Examples of radiation curable components are epoxides and(meth)acrylates. (Meth-)acrylates may comprise one or more reactivegroups for forming an acrylate polymer. The radiation curable medium maycomprise one type of radiation curable compound or alternatively, theradiation curable medium may comprise a mixture of radiation curablecompounds.

The radiation curable medium may further comprise at least oneinhibitor. An inhibitor is a component that prevent (inhibits) unwantedpolymerization of the radiation curable compound. Inhibitors may beadded to the radiation curable inkjet ink composition to increase theshelf life on the ink composition.

The radiation curable medium may further comprise at least one photoinitiator. A photo initiator is a component that improves the efficiencyof curing; i.e. increases the polymerization rate when the inkcomposition is irradiated with suitable radiation, such as UV radiation.

The radiation curable medium may further comprise a solvent, such aswater or an organic solvent. The solvent may be added to the radiationcurable medium to tune ink properties, such as viscosity.

Further, additional components may be added to the radiation curablemedium. For example, the radiation curable medium may comprisesurfactants, antibacterial components and anti-fungi components.

Colorant

The radiation curable inkjet ink composition may further a colorant,such as a pigment, a dye or a mixture thereof. Further, the radiationcurable inkjet ink composition may comprise a mixture of dyes and/or amixture of pigments. The colorant may provide the ink composition with apredetermined color.

Ester Compound

The radiation curable inkjet ink composition may further comprise anester compound. According to the present invention, the ester compoundconsists essentially of a condensation product of a first reactant and asecond reactant, wherein the first reactant is a compound A comprisingat least 3 first functional groups, and wherein the second reactantcomprises at least one compound B, wherein the at least one compound Bcomprises a second functional group, wherein the first functional groupis a first group selected from a hydroxyl functional group and acarboxylic acid functional group and the second functional group is asecond group selected from a hydroxyl functional group and a carboxylicacid functional group, wherein the first functional group is differentfrom the second functional group.

Hence, the ester compound may consist essentially of a condensationproduct of a first reactant and a second reactant, wherein the firstreactant is a compound A comprising at least 3 hydroxyl functional groupfunctional groups and wherein the second reactant comprises at least onecompound B, wherein the at least one compound B comprises a carboxylicfunctional group. Alternatively, the ester compound may consistessentially of a condensation product of a first reactant and a secondreactant, wherein the first reactant is a compound A comprising at least3 carboxylic acid functional groups and wherein the second reactantcomprises at least one compound B, wherein the at least one compound Bcomprises a hydroxyl functional group.

Preferably, compound B comprises only one second group. In case thesecond group is a hydroxyl functional group, the compound B preferablycomprises only one hydroxyl functional group. In case the second groupis a carboxylic acid group, the compound B preferably comprises only onecarboxylic acid group.

The ester compound formed by reacting the first reactant and the secondreactant may be a nonlinear ester compound. The ester compound maycomprise at least three ester groups. The ester compound may provide theradiation curable inkjet ink composition with gelling properties.

The ester compound may be used as a gelling agent in a radiation-curableinkjet ink composition for obtaining a glossy image.

An ink composition comprising an ester compound in accordance with thepresent invention may be capable of providing high gloss images. Theester compound may form a 3-dimensional network in the ink compositionupon cooling down, thereby changing the rheological properties of theink composition. Hence, when the droplets of ink cool down after beingjetted onto a recording medium, the viscosity of the ink may increaseand excessive flow of the ink droplets may be prevented, therebypreventing colorbleed.

The ester compound may be present in an amount of 0.3 wt %-3.0 wt %based on the total weight of the radiation-curable inkjet inkcomposition. Preferably, the ester compound may be present in an amountof 0.5 wt %-2.5 wt % based on the total weight of the radiation-curableinkjet ink composition, more preferably from 0.8 wt %-2.2 wt %, forexample from 1.0 wt %-2.0 wt %.

In case the ink composition comprises less than 0.3 wt % of the estercompound based on the total weight of the radiation-curable inkjet inkcomposition, then the increase in viscosity of the ink after printing onthe recording medium may be insufficient to prevent color bleeding.Hence, too little ester compound may result in decreased print quality.In case the ink composition comprises more than 3.0 wt % of the estercomponent, then the gloss level of the printed image may decrease.

In an embodiment, the ester compound does not comprise a (meth)acrylatefunctional group and/or a vinyl functional group. The ester compoundtherefore may not undergo a polymerization reaction upon irradiating theink composition with actinic energy radiation, such as UV radiation.

In an embodiment, gloss of an image may be determined using a micro-TRIgloss meter obtained from BYK-Gardner GmbH using the internalcalibration and measurement method. In case the gloss level measuredunder an angle of 60° is at least 35, preferably at least 40, the imagemay be considered to be a high gloss image. In case the gloss levelmeasured under an angle of 60° is less than 35, the image may beconsidered a low gloss image (matt).

In an embodiment, the compound A is selected from the group consistingof pentaerythritol, cyclodextrine, glycerol, dipentaerythritol,2-(hydroxymethyl)-2-methylpropane-1,3-diol,2-ethyl-2-(hydroxymethyl)propane-1,3-diol,2-(hydroxymethyl)propane-1,3-diol, trimethylolethane,trimethylolpropane, trimethylolbutane and trimethylolpentane.

These compounds are compounds comprising at least 3 hydroxyl functionalgroups. When reacted with a carboxylic acid, ester compounds can beformed.

Esters obtainable by reacting a carboxylic ester with a compound Aselected from the above listed group may be esters having a branchedstructure (i.e. non-linear esters). Without wanting to be bound to anytheory, it is believed that a branched structure may decrease thetendency of the ester compound to crystallize when cooling down. Hence,ester compound obtainable from the above mentioned polyalcoholcomponents may not crystallize when cooling down. This may improve thegloss of a print made with an ink composition comprising such estercompound.

Methods for synthesizing ester compounds starting from a compoundcomprising a plurality of hydroxyl functional groups and a compoundcomprising a carboxylic acid group are known in the art.

In an embodiment, the compound B is a compound according to formula I,wherein R is an alkyl group, an aryl group or an alkylarylgroup, whereinR is a group having 5-30 carbon atoms.

R—C(O)OH   formula I:

Compounds according to formula I are suitable to form ester compounds inaccordance with the present invention. The properties of the estercompound may be influenced by the choice of the functional group R. Thenature of the R group may for example influence the melting point of theester compound and the rate of diffusion of the ester compound in theinkjet ink composition. R may be an alkyl group, an aryl group or analkylarylgroup. When the functional group R comprises an aromatic unit,then pi-pi-interaction may occur. Pi-pi interaction may assist informing the intermolecular network upon cooling of the ink compositioncomprising the ester compound, which may be beneficial for the increasein viscosity of the ink composition when cooling down. The functionalgroup R may be a group comprising 5-40 carbon atoms, preferably 10-25.When the functional group R comprises less than 5 carbon atoms, theester compound may not show gelling behavior at printing conditions.When the functional group R comprises more than 40 carbon atoms, thenthe ester compound may not be fluid at jetting conditions, which mayhamper the jetting of the inkjet ink composition. The ester compound maycomprise only one type of functional group R. Alternatively, the estercompound may comprise a plurality of different R functional groups.

In a further embodiment, the compound B is a fatty acid. Fatty acids aresuitable for forming esters, when reacted with a compound comprising anhydroxyl functional group. The fatty acids may be saturated ornon-saturated fatty acids. Non-saturated fatty acids may bemonounsaturated fatty acids or polyunsaturated fatty acids.Non-saturated fatty acids comprise an alkene functional group. Uponcuring of the ink, the alkene functional group may react and the estercompound may be incorporated in the network formed by theradiation-curable component. Preferably, the fatty acid is a saturatedfatty acid. When the compound B is a fatty acid, no so-called bloomingof the ink may occur. Blooming is an unwanted phenomena that may occurin ink composition, such as radiation-curable ink composition comprisinga gelling agent. After being applied onto a recording medium, a gellingagent present in the ink may cool down and may solidify, thereby forminga three-dimensional network that increases the viscosity of the ink.However, in the course of time, the gelling agent may migrate to thesurface of the ink layer, which may result in matt print appearance. Thephenomenon of decreased gloss due to migration of the gelling agent isknown as “blooming”. Without wanting to be bound to any theory, it isbelieved that by selecting compound B to be a fatty acid, an amorphousester compound is obtained, that results in an ink composition that doesnot show blooming.

In a further embodiment, the ester compound ispentaerythritoltetrastearate. Pentaerythritoltetrastearate is an esterobtainable by reacting pentaerythritol and stearic acid. Stearic acid(CH₃(C₂)₁₆COOH) is a fatty acid.

In an embodiment, the radiation curable component is an acrylate havingtwo or more acrylate functional groups. An acrylate may undergo apolymerization reaction when irradiated by suitable radiation, such asUV radiation. Hence, a polyacrylate polymer may be formed when an inkjetink composition comprising an acrylate is cured, thereby hardening theink. An acrylate molecule having two or more acrylate functional groupsmay react with two or more other acrylate molecules and hence, apolymeric network may be formed. Examples of acrylates having two ormore acrylate functional groups are known in the art.

In a further embodiment, the ink composition further comprises amonofunctional acrylate. Presence of a monofunctional acrylate mayimprove the hardness and flexibility of the ink layer after curing.

In an embodiment, the ester compound is a gelling agent. The estercompound may be used as a gelling agent in a radiation-curable inkjetink composition for obtaining a glossy image.

An ink composition comprising an ester compound in accordance with thepresent invention may be capable of providing high gloss images. Theester compound may form a 3-dimensional network in the ink compositionupon cooling down, thereby changing the rheological properties of theink composition. Hence, when the droplets of ink cool down after beingjetted onto a recording medium, the viscosity of the ink may increaseand excessive flow of the ink droplets may be prevented, therebypreventing colorbleed.

In an aspect of the invention, a method for preparing aradiation-curable inkjet ink composition is provided, the methodcomprising the steps of:

-   -   providing a radiation-curable component;    -   providing an ester compound in accordance with the present        invention; and    -   mixing the radiation-curable component and the ester compound.

The radiation-curable component and the ester compound may be provided.Optionally, additional components may be provided, for example anadditional solvent. The radiation-curable component and the estercompound may be provided neat or they may be provided in a solution ordispersion. Optionally, a colorant may be provided. In case the colorantis a pigment, the pigment is preferably provided as a dispersion, suchas an aqueous pigment dispersion. The components may be provided atonce, or the components may be added subsequently. The components may beadded in any suitable order. In case a dispersible component is added(pigment and/or latex particles), such dispersible component may bepreferably added after the other components of the ink composition areprovided. Mixing of the components may be carried out at any suitabletemperature, for example room temperature.

In an aspect of the invention, a method for applying an image onto arecording medium is provided, the method comprising the steps of:

-   -   a. jetting droplets of a radiation-curable inkjet ink        composition according to the present invention onto the        recording medium;    -   b. curing the radiation-curable inkjet ink composition by        irradiating the ink composition using UV radiation.

In the method, an image is applied onto a recording medium. In themethod, in step a), an image is applied to the recording medium. Theimage may be applied using an ink composition according to the presentinvention. The ink composition may be applied onto the recording mediumin a predetermined fashion, e.g. in accordance with image files storedon suitable storing means. The image may be applied for example byjetting droplets of the radiation-curable inkjet ink composition usingan inkjet print head. The recording medium may be a sheet-like medium,such as a sheet of paper or a sheet of vinyl. Alternatively, therecording medium may be a web, for example an endless belt. The web maybe made of a suitable material. Optionally, the image may be dried afterit has been applied onto the intermediate transfer member.

In the method, in step b), the radiation-curable inkjet ink compositionis cured by irradiating the ink composition using UV radiation. Theinkjet ink composition may be irradiated using a suitable source ofradiation, such as a halogen lamp, a mercury lamp and/or a LED lamp.Optionally, a plurality of sources of radiation may be used to irradiatethe inkjet ink composition.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further features and advantages of the present invention areexplained hereinafter with reference to the accompanying drawingsshowing non-limiting embodiments and wherein:

FIG. 1A shows a schematic representation of an inkjet printing system.

FIG. 1B shows a schematic representation of an inkjet print head.

FIG. 2 shows the gloss level of a number of ink compositions as afunction of the wt % of gellant.

In the drawings, same reference numerals refer to same elements.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an ink jet printing assembly 3. The ink jet printingassembly 3 comprises supporting means for supporting an image receivingmedium 2. The supporting means are shown in FIG. 1A as a flat surface 1,but alternatively, the supporting means may be a platen, for example arotatable drum that is rotatable around an axis. The supporting meansmay be optionally provided with suction holes for holding the imagereceiving medium in a fixed position with respect to the supportingmeans. The ink jet printing assembly 3 comprises print heads 4 a-4 d,mounted on a scanning print carriage 5. The scanning print carriage 5 isguided by suitable guiding means 6 to move in reciprocation in the mainscanning direction X. Each print head 4 a-4 d comprises an orificesurface 9, which orifice surface 9 is provided with at least one orifice8, as is shown in FIG. 1B. The print heads 4 a-4 d are configured toeject droplets of marking material onto the image receiving medium 2.

The image receiving medium 2 may be a medium in web or in sheet form andmay be composed of e.g. paper, cardboard, label stock, coated paper,plastic or textile. Alternatively, the image receiving medium 2 may alsobe an intermediate member, endless or not. Examples of endless members,which may be moved cyclically, are a belt or a drum. The image receivingmedium 2 is moved in the sub-scanning direction Y over the flat surface1 along four print heads 4 a-4 d provided with a fluid marking material.

The image receiving medium 2, as depicted in FIG. 1A is locally heatedor cooled in the temperature control region 2 a. In the temperaturecontrol region 2A, temperature control means (not shown), such asheating and/or cooling means may be provided to control the temperatureof the receiving medium 2. Optionally, the temperature control means maybe integrated in the supporting means for supporting an image receivingmedium 2. The temperature control means may be electrical temperaturecontrol means. The temperature control means may use a cooling and/orheating liquid to control the temperature of the image receiving medium2. The temperature control means may further comprise a sensor (notshown) for monitoring the temperature of the image receiving medium 2.

A scanning print carriage 5 carries the four print heads 4 a-4 d and maybe moved in reciprocation in the main scanning direction X parallel tothe platen 1, such as to enable scanning of the image receiving medium 2in the main scanning direction X. Only four print heads 4 a-4 d aredepicted for demonstrating the invention. In practice an arbitrarynumber of print heads may be employed. In any case, at least one printhead 4 a-4 d per color of marking material is placed on the scanningprint carriage 5. For example, for a black-and-white printer, at leastone print head 4 a-4 d, usually containing black marking material ispresent. Alternatively, a black-and-white printer may comprise a whitemarking material, which is to be applied on a black image-receivingmedium 2. For a full-color printer, containing multiple colors, at leastone print head 4 a-4 d for each of the colors, usually black, cyan,magenta and yellow is present. Often, in a full-color printer, blackmarking material is used more frequently in comparison to differentlycolored marking material. Therefore, more print heads 4 a-4 d containingblack marking material may be provided on the scanning print carriage 5compared to print heads 4 a-4 d containing marking material in any ofthe other colors. Alternatively, the print head 4 a-4 d containing blackmarking material may be larger than any of the print heads 4 a-4 d,containing a differently colored marking material.

The carriage 5 is guided by guiding means 6. These guiding means 6 maybe a rod as depicted in FIG. 1A. Although only one rod 6 is depicted inFIG. 1A, a plurality of rods may be used to guide the carriage 5carrying the print heads 4. The rod may be driven by suitable drivingmeans (not shown). Alternatively, the carriage 5 may be guided by otherguiding means, such as an arm being able to move the carriage 5. Anotheralternative is to move the image receiving material 2 in the mainscanning direction X.

Each print head 4 a-4 d comprises an orifice surface 9 having at leastone orifice 8, in fluid communication with a pressure chamber containingfluid marking material provided in the print head 4 a-4 d. On theorifice surface 9, a number of orifices 8 are arranged in a singlelinear array parallel to the sub-scanning direction Y, as is shown inFIG. 1B. Alternatively, the nozzles may be arranged in the main scanningdirection X. Eight orifices 8 per print head 4 a-4 d are depicted inFIG. 1B, however obviously in a practical embodiment several hundreds oforifices 8 may be provided per print head 4 a-4 d, optionally arrangedin multiple arrays.

As depicted in FIG. 1A, the respective print heads 4 a-4 d are placedparallel to each other. The print heads 4 a-4 d may be placed such thatcorresponding orifices 8 of the respective print heads 4 a-4 d arepositioned in-line in the main scanning direction X. This means that aline of image dots in the main scanning direction X may be formed byselectively activating up to four orifices 8, each of them being part ofa different print head 4 a-4 d. This parallel positioning of the printheads 4 a-4 d with corresponding in-line placement of the orifices 8 isadvantageous to increase productivity and/or improve print quality.Alternatively multiple print heads 4 a-4 d may be placed on the printcarriage adjacent to each other such that the orifices 8 of therespective print heads 4 a-4 d are positioned in a staggeredconfiguration instead of in-line. For instance, this may be done toincrease the print resolution or to enlarge the effective print area,which may be addressed in a single scan in the main scanning directionX. The image dots are formed by ejecting droplets of marking materialfrom the orifices 8.

The ink jet printing assembly 3 may further comprise curing means 11 a,11 b. As shown in FIG. 1A, a scanning print carriage 12 carries the twocuring means 11 a, 11 b and may be moved in reciprocation in the mainscanning direction X parallel to the platen 1, such as to enablescanning of the image receiving medium 2 in the main scanning directionX. Alternatively, more than two curing means may be applied. It is alsopossible to apply page-wide curing means. If page-wide curing means areprovided, then it may not be necessary to move the curing means inreciprocation in the main scanning direction X. The first curing means11 a may emit a first beam of UV radiation, the first beam having afirst intensity. The first curing means 11 a may be configured toprovide the radiation for the pre-curing step. The second curing means11 b may emit a second beam of radiation, the second beam of radiationhaving a second intensity. The second curing means 11 b may beconfigured to provide the radiation for the post-curing step.

The carriage 12 is guided by guiding means 7. These guiding means 7 maybe a rod as depicted in FIG. 1A. Although only one rod 7 is depicted inFIG. 1A, a plurality of rods may be used to guide the carriage 12carrying the print heads 11. The rod 7 may be driven by suitable drivingmeans (not shown). Alternatively, the carriage 12 may be guided by otherguiding means, such as an arm being able to move the carriage 12.

The curing means may be energy sources, such as actinic radiationsources, accelerated particle sources or heaters. Examples of actinicradiation sources are UV radiation sources or visible light sources. UVradiation sources are preferred, because they are particularly suited tocure UV curable inks by inducing a polymerization reaction in such inks.Examples of suitable sources of such radiation are lamps, such asmercury lamps, xenon lamps, carbon arc lamps, tungsten filaments lamps,light emitting diodes (LED's) and lasers. In the embodiment shown inFIG. 1A, the first curing means 11 a and the second curing means 11 bare positioned parallel to one another in the sub scanning direction Y.The first curing means 11 a and the second curing means 11 b may be thesame type of energy source or may be different type of energy source.For example, when the first and second curing means 11 a, 11 b,respectively both emit actinic radiation, the wavelength of the radiatedemitted by the two respective curing means 11 a, 11 b may differ or maybe the same. The first and second curing means are depicted as distinctdevices. However, alternatively, only one source of UV radiationemitting a spectrum of radiation may be used, together with at least twodistinct filters. Each filter may absorb a part of the spectrum, therebyproviding two beams of radiation, each one having an intensity differentfrom the other.

The flat surface 1, the temperature control means, the carriage 5, theprint heads 4 a-4 d, the carriage 12 and the first and second curingmeans 11 a, 11 b are controlled by suitable controlling means 10.

Experiments and Examples Materials

SR 9003 (propoxylated neopentyl glycol diacrylate) was obtained fromSartomer. MEHQ (monomethylethyl of hydraquinone) was obtained from SigmaAldrich. Stearone was obtained from Alfa Aesar.Pentaerythritoltetrastearate was obtained from NOF Corporation.Irgacure® 379 was obtained from BASF. ITX (2-isopropylthioxanthone) wasobtained from Rahn. All chemicals were used as received.

Methods

Gloss

The gloss of an image was measured after the image had been printed andcured. The gloss was measured using a micro-TRI glossmeter obtained fromBYK-Gardner GmbH using the internal calibration and measurement method.The micro-TRI gloss measuring device simultaneously measures the glossunder an angle of 20°, 60° and 85°, respectively. The gloss levelreported is the gloss level measured under an angle of 60°. A high valuerelates to a high gloss level, a low value relates to a low gloss level(matt).

Rodcoating

Rodcoats were made by applying a 14 μm thick layer of ink onto areceiving medium. As receiving medium, Avery Dennison MPI2000 was used.MPI2000 is a self-adhesive vinyl medium.

The ink was cured by irradiating the ink layer using a LED lamp emittingradiation having a wavelength of 395 nm.

Examples

Several ink compositions were prepared. Ink composition Ex 1-Ex 3comprises pentaerythritoltetrastearate as a gelling agent, in an amountof 1.0 wt % with respect to the total ink composition and is an inkcomposition according to the present invention. Ink composition CE 1comprises stearone as a gelling agent and is not ink compositionaccording to the present invention.

Production Example Ex 1

Ink composition Ex 1 was prepared by adding 100 grams of SR9003, 5.0 gof Irgacure® 379, 5.0 gram of ITX and 0.1 gram of MEHQ and 0.5 gram ofpentaerythritoltetrastearate to a flask and mixing the ingredients. Acolorless ink composition Ex 1 was obtained.

Ink compositions Ex 2, Ex 3 and CE 6 were prepared analogously, butdifferent amounts of pentaerythritoltetrastearate were used, as shown intable 1.

Comparative Example CE 1

The comparative ink composition CE 1 was prepared in a similar waycompared to Ex 1. However, no pentaerythritoltetrastearate was used whenpreparing comparative ink composition CE 1.

The comparative ink composition CE 2 was prepared in a similar waycompared to Ex 1. However, 0.5 gram of stearone was used when preparingcomparative ink composition CE 1, instead of 0.5 gram ofpentaerythritoltetrastearate. Ink compositions CE 3, CE 4 and CE 5 wereprepared analogously, but different amounts of stearone were used, asshown in table 1.

The comparative ink composition CE 6 was prepared in a similar waycompared to Ex 1, but a different of pentaerythritoltetrastearate wasused, as shown in table 1.

TABLE 1 Examples and Comparative examples wt % Amount of Amount of theInk pentaerythritoltetra- of gelling compostion: stearate: stearoneagent Ex 1 0.5 0 0.45 Inv Ex 2 1.0 0 0.90 Inv Ex 3 2.0 0 1.8 Inv CE 1 00 0 Comp CE 2 0 0.5 0.45 Comp CE 3 0 1.0 0.90 Comp CE 4 0 2.0 1.8 CompCE 5 0 5.0 4.3 Comp CE 6 5.0 0 4.3 Comp

The wt % of the gelling agent shown in table 1 corresponds to the weightpercentage of gelling agent present in the ink composition based on thetotal weight of the ink composition.

Rodcoats were made using ink compositions Ex 1-Ex 3 and CE 1-CE 6. Thegloss of the rodcoats provided with the (cured) ink compositions wasmeasured. The results are summarized in table 2 and FIG. 2.

TABLE 2 Gloss measurements Ink compositions Gloss Ex 1 64 Ex 2 54 Ex 342 CE 1 89 CE 2 34 CE 3 29 CE 4 12 CE 5 5 CE 6 31

When comparing the gloss levels of rod coats made with Ex 1-Ex 3 and CE6 on the one hand to CE 2-CE 5 on the other hand, it is observed thatthe ink compositions comprising x wt % of pentaerythritoltetrastearateas gelling agent show higher gloss than ink compositions comprising x wt% of stearone as a gelling agent. For example, the gloss level of therodcoat made with ink composition Ex 1—which is an ink compositionaccording to the present invention—was higher than the gloss level ofthe rodcoat made with ink composition CE 2, which is not an inkcomposition according to the present invention.

Rod coats made with ink composition CE 1, which does not comprise agelling agent, show higher gloss than the other rod coats. However,because ink composition CE 1 is free of gelling ink, no gelling behaviorwill occur when cooling down the ink. When the ink composition isprinted onto a receiving medium, no significant increase in viscositywill occur and hence, the droplets of this ink composition will spreadover a relatively large area of the recording medium. This may lead e.g.to color bleeding which results in bad print quality and is thereforeunwanted. Therefore, ink composition CE 1 may not provide images havingsufficient print quality, when a printing device as shown in FIG. 1A isused to form the image.

Further, it is observed that the gloss decreases upon increasing amountof gelling agent. The more gelling agent is present in the inkcomposition, the lower the gloss level of images made with the inkcomposition. However, the decrease in gloss is much lower for inkcompositions comprising pentaerythritoltetrastearate than for inkcompositions comprising stearone. Therefore, when using a gelling agentin accordance with the present invention in a low amount (i.e. from 0.3wt %-3.0 wt %), a UV curable gelling ink may be provided that allowsobtaining images having high gloss.

Hence, using ink compositions according to the present invention, highgloss levels can be obtained.

Detailed embodiments of the present invention are disclosed herein;however, it is to be understood that the disclosed embodiments aremerely exemplary of the invention, which can be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually andappropriately detailed structure. In particular, features presented anddescribed in separate dependent claims may be applied in combination andany combination of such claims are herewith disclosed. Further, theterms and phrases used herein are not intended to be limiting; butrather, to provide an understandable description of the invention. Theterms “a” or “an”, as used herein, are defined as one or more than one.The term plurality, as used herein, is defined as two or more than two.The term another, as used herein, is defined as at least a second ormore. The terms including and/or having, as used herein, are defined ascomprising (i.e., open language). The term coupled, as used herein, isdefined as connected, although not necessarily directly.

1. A radiation-curable inkjet ink composition comprising a radiationcurable component, the radiation-curable inkjet ink composition furthercomprising an ester compound, the ester compound consisting essentiallyof a condensation product of a first reactant and a second reactant,wherein the first reactant is a compound A comprising at least 3 firstfunctional groups, and wherein the second reactant comprises at leastone compound B, wherein the at least one compound B comprises a secondfunctional group, wherein the first functional group is a first groupselected from a hydroxyl functional group and a carboxylic acidfunctional group and the second functional group is a second groupselected from a hydroxyl functional group and a carboxylic acidfunctional group, wherein the first functional group is different fromthe second functional group and wherein the ester compound is present inan amount of 0.3 wt %-3.0 wt % based on the total weight of theradiation-curable inkjet ink composition.
 2. A radiation-curable inkjetink composition according to claim 1, wherein the compound A is selectedfrom the group consisting of pentaerythritol, cyclodextrine, glycerol,dipentaerythritol, 2-(hydroxymethyl)-2-methylpropane-1,3-diol,2-ethyl-2-(hydroxymethyl)propane-1,3-diol,2-(hydroxymethyl)propane-1,3-diol, trimethylolethane,trimethylolpropane, trimethylolbutane and trimethylolpentane.
 3. Aradiation-curable inkjet ink composition according to claim 1, whereinthe compound B is a compound according to formula I;R—C(O)OH;   formula I: wherein R is an alkyl group, an aryl group or analkylarylgroup, wherein R is a group having 5-30 carbon atoms.
 4. Aradiation-curable inkjet ink composition according to claim 3, whereinthe compound B is a fatty acid.
 5. A radiation-curable inkjet inkcomposition according to claim 2, wherein the ester compound is a fattyacid ester of pentaerythritol.
 6. A radiation-curable inkjet inkcomposition according to claim 5, wherein the ester compound ispentaerythritoltetrastearate.
 7. A radiation-curable inkjet inkcomposition according to claim 1, wherein the radiation curablecomponent is an acrylate having two or more acrylate functional groups.8. A radiation-curable inkjet ink composition according to claim 7,wherein the ink composition further comprises a monofunctional acrylate.9. A radiation-curable inkjet ink composition according to claim 1,wherein the ester compound is a gelling agent.
 10. A method forpreparing the radiation-curable inkjet ink composition according toclaim 1, the method comprising the steps of: providing a radiationcurable component; providing an ester compound, the ester compoundconsisting essentially of a condensation product of a first reactant anda second reactant, wherein the first reactant is a compound A comprisingat least 3 first functional groups, and wherein the second reactantcomprises at least one compound B, wherein the at least one compound Bcomprises a second functional group, wherein the first functional groupis a first group selected from a hydroxyl functional group and acarboxylic acid functional group and the second functional group is asecond group selected from a hydroxyl functional group and a carboxylicacid functional group, wherein the first functional group is differentfrom the second functional group; and mixing the radiation curablecomponent and the ester compound.
 11. A method for applying an imageonto a recording medium, the method comprising the steps of: a. jettingdroplets of the radiation-curable inkjet ink composition according toclaim 1 onto the recording medium; and b. curing the radiation-curableinkjet ink composition by irradiating the ink composition using UVradiation.